Test apparatus for measuring time delays



Dec. l, 1959 v. F. cARTwRlGHT TEST APPARATUS FOR MEASURTNG TIME DELAYs med oct. 1v, 1955 2 Sheets-Sheet l A TTORNEVS v. F. cAR-rwRlGHT 2,915,700

TEST APPARATUS FOR MEASURING TIME DELAYS 2 Sheets-Sheet 2 Ftpkt,=2o/s Dec. l, 1959 Filed oct. 17, 1955 TOTAL T/M' DLV- /1 5 INVENTOIL v/croR i cARTwR/GHT @e l? L a ArrQR/vEl/s TEST APPARATUS Fon MEASURING DELAYS This invention-relates to improved apparatus for `measuring the time delay and for measuring variations in the time delay of an electric signal through various types of electrical circuits.

It is desirableto ascertain-the-time delay of an electric signal through many types of` electrical circuits. vBy way of example, measurement of Vsuch time delays is desirable for many lumped constant circuits, delay lines, `video amplifiers, limiters, radio frequency band-pass circuits, radio frequency amplifiers, and radio receivers. The measurement of the time delay through such an electrical circuit may be for the purpose of ascertaining the total delay which is introducedby the circuit, or for the purpose of ascertainingthe effect of certain variables upon the operation of the circuit. -For example, it may be desirable to ascertain variations in the time delay which Z,9l5,700 Patented Dec. l, 1959 make measurements having an accuracy of plus or minus .001 v microsecond.

The invention is explained in detail with reference to the drawings, in which:

Fig. 1 is a block diagram illustrating a preferred embodiment of the invention; q

Fig. 2 illustrates a modification of the apparatus for providing more precise frequency measurements;

Fig. 3 shows the wave forms of various signals which are produced in the circuits of theapparatus of Fig. l, and the letters which are employed to designate the wave forms are also employed to designate the location in the apparatus of Fig. 1 where the signals occur; and

Fig. 4 is a curve showing how the oscillating frequency of the apparatusof Fig. 1 varies with changes in the total time'delay of the signal through the apparatus.

The measurement apparatus of the Ainvention may be provided as asingle unit to which the test specimen is are caused by temperatureV variations, shock and yvibration, the size of moisture and protective coating materials, electrode potential changes, variations in the amplitude of input signals, variations in the frequency of input signals, variations in the atmospheric pressure, and the eiects of detuningvstages of a-multi-stage circuit.

Heretofore, complex arrangements `have been employed to make such measurements. As a result, the measurements are difficult to obtain, and the measurements are ordinarily limited -to measurement ofthe delay through an entire system,vsince it would require considerable time and eifort to measure the time delay of the signal through'theindividual componentsof the system. Y,

These difficulties are overcome inthe present invention by providing .a simple arrangement yfor measuring 'the time delayv ofvan electrical signal through component generator are applied through the specimen to betested to the input 'ofthe' signall generating means so as to cause the signal generating means to produce pulse signals having a repetition frequency which is ,governed bythe time delay introduced by the specimen. Measurement of the repetition frequency of the signal which is. produced by the signal generator provides a measure of the 'time delay introduced by the specimen.

l The pulse signals may be unidirectional signals-for testing components such as video amplifiers and ,ther like, orqthey maybe radio frequency pulse signals for testing components such as radio frequency ,amplifiers and-.the like. If desired, therise timeof the pulse signals may be adjusted so as to effect measurementover a wide frequency spectrum'. v

Thus, the apparatus ofthepresent inventionmay be employed in A,testing ymany types of Aelectrical circuits, the lapparatus is easy to operate since it merely requiresl a frequency measurement, and the apparatus isextremely yvith great precision. By way of example, it is' leasy. to

coupled in order to make the desired measurements. In the alternative, .the test apparatus of the invention may be in the form-of a basic vunit which may be coupled to auxiliary equipment which is ordinarily yavailable in most laboratories. ,Such ,a basic vunit is designated by the numeral 10 in the apparatus of Fig. V1. .A The basic unit comprises a video amplifier y12, a limiter 14, a delayl circuit 16, and a pulse signal generator 18.

Preferably the video amplifier, the limiter, and the delay circuit are Wide-band types. Also, it is preferable that the amplifier have an even .number of stages so that the output of the amplieris in phase with its input. The limiter provides an output signal which is independent of noise or other variations' in the signalwhich is applied to its input and the limiter serves tofincrease the stability of operation of thel .test apparatus. generator 18 may be a blocking oscillator or a Inu-tivibrator arranged to produce an output pulse in response to each pulse signal which ,is-applied to-its input circuit. If desired, the pulse signal ,generator may be arranged to provide pulse signals of adjustable width4 s o `as yto permit measurement of the time delay of pulses of various widths through the test specimen.y Also, gthelpulsersignal generator, or an auxiliary circuit -Coupled-,to lits output signal generator vto produce the first pulse, and thereafter' the successive pulses are produced in response to =the ,signals whichare applied through -the testnapparatus to the input of the pulse signal generator.

Frequency measurement apparatus 22 is coupled to the output of the pulse signal generator for measuring Ithe repetition frequency ofthe signals which are produced by the pulse signal generator.

In the testsetup which is illustratedin Fig. l, theA output ofthe basic unit `V10 lis applied'through an adjustable attenuatorf24v to a Vswitch 26. The `switch has contacts 27,28 and 29 for providing connectionsto various parts of the circuit. The switch contact A2'7 is connectedto an adjustable attenuator 30 which in turn is connected to the input of the video'amplier 12. The switch contact 28fis connected to the input of a test specimen 32, and the output of thev test specimen may be coupled to the input of-theadjustable attenuator 36). The'switch contact 29 is .connected to a radio frequency pulse `generator 34, land the output of the radio frequency pulseA generator is connected to vthe'input of the test specimen. A switch 36 is .provided for connecting the output of the testvspecimen through a switch contact- 37 `to the adjustable attenuator 30 or through Vthe switch contact 38 to a radio frequency load and detector 40.' The outputof the radio 'Ihe pulse signal Y 3 frequency load and detector is coupled attenuator 30. t i f Preferably the entire pulse system should be of the wideband variety so that the pulses will rise and decay rapidly and so` .that the `.lowest frequency component of the'pulse (i.e. the oscillating frequency), VWill'n'otbe altered in time by the oscillating frequency. Since a wide,- band system is prone lto harmonic oscillations, it is desirable 'to prevent undesired or spurious operation of the pulse signal generator. I prefer'to do this `by employing a monostable multivibrator 42 "to control the action ofthe pulsefsignal generator. The monostable multivibrator 42 is responsive to the pulses which are produced by the pulse signal generator'18 and it disables the pulse signal generator for a predeterminedy `time after each output signal is produced so yas to prevent spurious operation ofthe' pulse signal generator. Preferably,`th`e monostable multivibrator disablesithei pulse signal generator after each output pulse is produced for a time *which is slightly less thanthe time delay ofthe delay lcircuit `16'.

lto the adjustable This disabling action may be effected by a conventional gating arrangement which servesto gate the pulse signal generator on and off atthe desired times.

It will be apparent that `other arrangements maybe employed for preventing spurious operation of the pulse signal generator. paratus may be disabled by use of a gate at other locations yinthe circuit to prevent the apparatus from conveying a pulse for a predetermined time after each out put pulse is produced by thepulse signal generator.y

If desired, a peak voltmeter 44 may be employed either v at the output or at the input of the video amplifier 12 for measuring the amplitude of the pulse signals which are employed to` trigger the pulse signal generator 18. Ordinarily it is desirable to maintain the triggering signals at substantially constant amplitude.

Also, an audio signal may be produced so as to provide For example, rthe measurement apf a rough indication of thedelay produced by the testV specimen.' Such an arrangement is illustrated in Fig. 1 wherein a heterodyne oscillator and mixer 46 Vis coupled to the output of the video amplifier. The output of the heterodyne oscillator and mixer is applied through an amplifier 48 to a loud speaker 50.

The measurement apparatus may be termed a delay control oscillator. The frequency of the oscillations may be expressed as follows:

where t1 equals'the delay of all of the circuitry except the test specimen and :3 is the delay introduced by the `test specimen.

The amount of frequency change may be found by differentiating the above equation, which gives the following result:

`By Way of example, if trl-t2 equals 20 microseconds, the oscillating frequency will be 50,000 c.p.s., and this frequency will vary 2.5 c.p.s. per .001 microsecond change in delay. A typical curve for such operation is illustrated in Fig. 4. p i y In operation, the switch 26 may bes connected to the terminal 27 to ascertainV the oscillating frequency of the circuit Without the -test specimen. The switch 20 is actuatedto cause the pulse signal generator to produce a rst pulse, yand `this pulse is applied through the adjustable attenuator 24, the switch terminal V27, the adjustable attenuator 30, `the video amplifier 12, the limiter 14, and the delay circuit 16 `to theinput of the pulse signal generator. The pulse signal generator produces an output pulse `in response to each input pulse, and `hence the oseillatingor repetition frequency for this condition of operation 1s determined by theptime delay which is intro- 1-1 g 4 if duced by `ther delay circuit 16 plus anyi other time delays which may beinherentin the circuit. I 't In order to measure the time delay which 1s 1ntroduced by the test specimen 32, the switch 26 is connected to the terminal 28, and the switch 36 is connected to the terminal 37. The wavey forms of Fig. 3 illustrate typical signals which may be generated in various parts of the circuit of Fig.'1 for this condition of operation. The rstpulse of thewave forms A is producedby actuating switch 20 and thereafter the pulse signal generator produces the pulses in response to the signals C which are applied to its. input circuit. `The pulses A are applied through the test specimen 32 to provide pulses B and its output circuit which are delayed a time t2 vbythe time delay which isfintroduced by the test specimen. In the arrangement illustrated, the time delay which is caused. by the delay circuit 16 and the remainder of the circuit other thanv the'test specimen is 20 microseconds, and the pulsesC which `are applied Ito the input ofthe pulse signal generatorr are delayed 20 microseconds with respect to the pulses B which are produced at the output of the test specimen. `The monostable multivibrator 42 provides a control signal D which disables the pulse signal generatorV 18 for 18 microseconds after each pulse so lasto prevent spurious operation of the pulse signal generator.

Thus, the periodicity or repetition rate of the pulses which are produced bythe pulse signal generator 18 is determined by the sum of thetiume delays t1 and t2 produced -by the delay circuit and the test specimen. j

In the example illustrated in the drawings, the r-oscillating frequency is 50,000" c.p.s. whenthe switch 26 is If a radio frequency signal is to be conveyed through.

the test specimen, the switch `26 is connected to the contact 29 so that the pulses whieh are produced by the pulse signal generator 18 actuate the radio `frequency pulse generatorr 34. The switch 36 is connected to the contact 38 so that the radio frequency pulses which are conveyed through the test specimen 32 are applied to the radio frequency load and detector 40. The radio frequency load and detector serves to produce unidirectional pulse signals which are applied to the adjustable attenuator 30 so that they will trigger the pulse signal gen erator 18. j

Varioustypes of frequency measuring apparatus may be employed. A conventional `device foi providing an indication of frequency or forproviding an indication of events per unit of `time may be employed. Also, a frequency discrminator may be employed to drive a calibrated recorder so as to provide a continuous record of time` delays over a period of time.` i

Since the interval of` one oscillating cycle is determined by t1+t2, if`we divide the frequency by a fixed number (N) the result of the divided or counted-down interval is Ntl-l-Ntz and therefore t2 is effectively multiplied by the division factor N. Thus, if t2 varies .001 microsecond and the frequency is divided by 100, the divided interval changes .001.times microsecond, which is equal to 0.1 microsecond.` This effect is equivalent to magnifying the delay t2 by a factor of 100. p

Fig. 2 illustrates an arrangement for providing such magnification. In this arrangement, the output signals A which are produced by the pulse signal generator 18 are applied to a frequency divider 60 which produces a divided version of the signal at its output circuit. This divided signal may be applied to' various types of frequency measurement apparatus such as an oscilloscope or a puls-Width type discn'minator.

if desired.

The radio frequency generator 34 may provide radio frequency pulse, amplitudeLmodulated signals, 'or frequency-modulated signals, as required for the particular specimento be tested.

1. In apparatus for measuring the time delay of a signal through a specimen, signal generating means for producing 'a discrete'outpt signal 'in response to an `i`n put signal, means for applying the output signals of the signal generating means through the specimen to be tested to the input of the signal generating means to cause the signal generating means to produce signals having a repetition frequency which is governed by the time delay introduced by the specimen, means coupled to the out-put of the pulse generating mean-s for disabling the measurement apparatus for a predetermined time after each output si-gnal is produced to prevent spurious operation of the signal generating means, and means coupled to the output of the signal generating means for measuring the repetition frequency of the signals produced by the signal generating means to provide a measure of the time delay caused by the specimen.

2.v In apparatus for measuring lthe time delay of a signal through a specimen, signal generating means for producing an output signal in response to an input signal, a delay circuit for delaying an input signal a fixed time, means for applying the output signals of the signal generating means through the specimen to be tested and through the delay circuit to the input of the signal generating means to cause the signal generating means to produce signals having a frequency which is governed by the sum of the time delays produced by the delay circuit and the test specimen, and means coupled to the output of the pulse generating means for. disabling the signal generating means after each output signal is produced for a period of time which is slightly less than the time delay of the delay circuit to prevent spurious operation of the signal generating means.

3. Measurement apparatus comprising signal generabing means for producing a pulse signal in response to an input signal, a test specimen and a delay circuit coupled in series between the output of the signal generating means and the input of the signal generating means for causing the signal generating means to produce pulse signals having a repetition frequency which is governed by the sum of the time delays caused by the delay circuit and the test specimen, the time duration of the respective pulse signals being a small fraction of the time delays caused by the delay circuit and the test specimen, means coupled to the output of the signal generating means for disabling the measurement apparatus for a predetermined time after each output signal is produced to prevent spurious operation of the signal generating means, and means responsive to the pulses generated by said signal generating means for measuring the repetition frequency of the signals produced by the signal generating means to provide a measure of the time delay caused by the test specimen.

4. The apparatus of claim 3 wherein the signal gen erating means is a pulse generator which produces unidirectional pulses.

5. The apparatus of claim 3 wherein the signal gen` erating means includes a radio frequencyy signal .generator for producing radio frequency pulse signals.

6, The apparatus of claim 3 wherein the means for Vs' l disabling the measurement apparatus "is -a' monostable multivibrator'responsive tothe pulse 'signals produced by thewsig'nal generating means and having its output coupled tothe "signal generating means for disabling thek l signal generating means after each output signal is producedffor a period of time which isfslightlyless than the 'timev delayof the delay circuit.

l7.'The"apparatus of'claim 3 wherein the means foi measuring the repetition lfrequency 'of thev signals pro` duced by the signal generating ineansl includes a frequency divider for enhancing the accu'racyofthe measurement. v

8. Time-delay measurement apparatus comprising a video amplifier having an input circuit for'receiving pulse signals, a limiter coupled to the output of the video ampliiier, afpulse signal generator coupled to the output Vof the limiter for producing a' 'pulse' signal at its output circuit in response to an input signal, so that a test specimen may be coupled between the output circuit of the pulse signal generator and the input circuit of the video amplier to cause the pulse signal generator to produce signals having a repetition frequency which is governed by v the time delay caused by the test specimen, and means coupled to the output of the signal generator for disabling the measurement apparatus for a predetermined time after each output signal is produced by the pulse signal generator to prevent spurious operation of the pulse signal generator.

k9. Time-delay measurement apparatus comprising a video amplifier having an input circuit for receiving pulse signals, a limiter coupled to the output of the video ampliiier, a delay circuit coupled to the output of the limiter, a pulse signal generator coupled to the output of the delay circuit for producing a pulse signal at its output circuit in response to an input signal, so that a test specimen may be coupled between the output circuit of the pulse signal generator andthe input circuit of the Video amplifier to cause the pulse signal generator to produce signals having a repetition freqency which is governed by the sum of the time delays caused by the delay circuit and the test specimen, and means coupled to theoutput of the signal generator for disabling the pulse signal generator after each output signal is produced for a period of time which is slightly less than the time delay of the delay circuit to prevent spurious operation of the signal generating means.

10. Measurement apparatus comprising a signal generator for producing a pulse signal in response to an input signal, a test specimen coupled to the output of the signal generator, a video amplifier coupled to the output of the test specimen, a limiter coupled to the output of the video amplifier, a delay circuit coupled between the output of the limiter and the input of the signal generator to cause the signal generator to produce signals having a repetition frequency which is governed by the sum of the time delays caused by the delay circuit and the specimen, and means responsive to pulses generated by said signal generator for measuring the frequency of the signals produced by the signal generator.

11. The apparatus of claim l0` further including means coupled to the output of the Video amplifier for measuring the amplitude of the pulse signals which are applied to the limiter.

l2. The apparatus of claim l0 further including means coupled to the measurement apparatus for providing an audio signal having a frequency which is representative of the repetition frequency of the pulse signals produced by the signal generator.

13. Measurement apparatus comprising a pulse signal generator for producing a pulse signal in response to an input signal, a radio frequency signal generator coupled to the output of the pulse signal generator for producing a radio frequency pulse signal in response to each signal produced by the pulse signal generator, a test specimen coupled to the output of the radio frequency signal .generator, and means including a delay circuit coupling the output of the test specimen to the input of the pulsesignal generator for causing the pulse signal generator to' produce pulse signals having a repetition frequency which is governed by the sum of the time-delays `caused by the delay circuit and the test specimen.

`14. Measurement apparatus comprising a; pulse signal generator for producing a pulseL signal in responsetdan input signal, a radio `frequencyfsi'gnal generator coupled to the output 'of the pulse signal generator for producing a radio frequency pulse signal in response to each` pulse produced by the pulse signal generator, a test specimen coupled to the output of the radio` freqency signal generator, a video amplifier coupled to the output of the` test specimen, a limiter coupled to the output ofthe video amplifier, a delay circuit coupled between `the, outputof the limiter and the input of the signal generator to cause the pulse signal generator to produce signals `having a repetition frequency which is governed by the sum of the time delays caused by the delay circuit and the specimen, and means responsiveto pulses generated by the signal generator for'measuring 'therepetition frequency of the signals produced by` thepulse signal generator.

RefereucestCited in the file of this patent "UNITED STATES PATENTS 

